Stepped reaction piston for hydraulic pump



Aug; 12, 196.9 1 fr, CQLUNS I 3,460,484

STEPPED REACTION PISTON FOR HYDRAULIC PUMP Filed July 6, 1967 2 Sheecv-Sheet `1 IN VNY'OR AUS- 12, 41969 .1. F. coLLlNs 3,460,484

` STEPPED REACTION PISTON FOR HYDRAULIC PUMP Filed .my e. 19e? 2 sheets-sheet, z

United States Patent 3,460,484 STEPPED REACTEN PISTN FOR HYDRAULIC PUMP Joseph F. Collins, Butler, Ind., assignor to The Weatherhead Company Filed `l'uly 6, 1967, Ser. No. 651,512 Int. Cl. Ftlib 1/16, 1.7/00

U5. Cl. 163-1173 9 Claims ABSTRACT F THE DISCLOSURE A positive displacement pump of the axial piston type in which the output displacement is varied by axial sliding movement of the cylinder block. The duid is conducted from each of the cylinders by a tubular reaction piston which communicates with a check valve assembly in the housing and the fluid passes through the check valve .assembly into a common outlet. The reaction piston is formed with a number of stepped portions, of progressively decreasing diameter away from the cylinder with the smallest diameter being at the end adjacent the check valve assembly.

SUMMARY OE THE NVENTION This invention relates generally to pumps and more particularly to the reduction of the hydraulic noise level in positive displacement pumps.

Many efforts in refining and improving hydraulic systems, particularly in certain environments have been directed toward reducing the noise that occurs in the system. In piston type variable displacement pumps one of the sources of noise apart from the mechanical driving arrangement, where a certain amount of noise is created in the bearings and from the forces involved in reciprocating the pistons, is the fact that pumping action of the piston on the fluid being pumped creates a pulse or surge throughout the uid system. While this noise occurs at a basic frequency or ripple frequency depending on the total rate of piston oscillations which is usually the number of pistons multiplied by the rotational speed of the driving cam, it has been found that noise occurs at other frequencies in addition to the basic ripple frequency.

In analyzing this hydraulic noise, it has been found that various portions of the hydraulic system have different resonant frequencies and the fluid within these portions of the system can be oscillated with the pulsating flow of huid at the ripple frequency acting as a driver to produce noise at many other frequencies in addition to the ripple frequency. Because the intensity of the ripple frequency noise is at its greatest in the pumping cylinder, the noise generated in the pumping cylinder and between the cylinder and, in the case of a multiple piston pump, at the outlet manifold or co1- lection chamber in which the output of the individual cylinders is combined, is the source of a major portion of the hydraulic noise.

According to the present invention it has been found out that the fundamental ripple frequency, that is the number of pistons times the rotational speed of the input shaft, does not constitute an important portion of the objectionable noise. This is in part due to the fact that this noise is of a relatively low frequency below that to which the human ear is most sensitive and is also partially damped particularly in the case of pumps having a large number 0f pistons by the overlapping nature of the impulses into the outlet manifold. In addition, these lower frequencies are more easily damped by the remainder of the pump structure and conduit Patented Aug. 12, 1969 lines so as not to be effectively transformed into airborne noise.

However, it is found that the frequency of the pulsation in an individual cylinder, that is, the frequency which occurs at the same frequency of the drive shaft 1s an important source of noise because the higher harmonics of this basic frequency tend to be particularly strong and are often of considerably greater energy and sound level than the basic frequency.

For the foregoing reasons, it has been possible to consider, for purposes of noise reduction each individual cylinder as a separate noise generator and therefore to minimize t-he production of the higher harmonic frequencies of the basic drive shaft frequency or piston cyclic frequency. This is most effectively accomplished by utilizing the reaction piston to provide an acoustic impedance, since such an impedance at the point where the duid ows through the impedance has the greatest effect in reducing the amplitude of the most undesirable harmonics. By forming the interior passage through the reaction piston with a series of cylindrical portions of successively reduced diameter away from the face of the piston, shoulders are formed at distances such as to effectively approximate the shape of a negative exponential horn. While shaping the passage through the reaction piston with the shape of a negative exponential horn is highly effective, such a shape is extremely expensive and dicult to machine and it has been found that the step-like approximation is considerably cheaper to manufacture and retains a high degree of the effectiveness of the negative exponential horn. In addition, it may be desirable to provide a venturi portion beyond the smallest restriction in the reaction piston so as to result in another change in velocity of the duid as it reaches the check valve assembly.

It is believed that the construction :according to the present invention is effective to reduce noise in part because it results in a differential velocity of the fluid through the reaction piston because of the changing effective diameter of the passage through which the ffuid must pass. Thus, as tire passage is reduced in diameter, the velocity of the fiuid must increase proportionally and this change in velocity in part affects the ability of the hydraulic uid to generate and conduct pressure waves at a frequency which may be resonant between certain surfaces of the space defined by the reaction piston, the pumping piston and the cylinder. In addition, the reduction in diameter in the reaction piston tends to form a refiective surface defining the resonant frequency which is constantly changing as the piston moves within the cylinder For this reason the potential resonant frequencies within the cylinder are constantly changing as the piston moves on the pumping stroke, which is the time during which the pressure waves are created which when reinforced in a resonant manner result in objectionable hydraulic noise.

lt has also been found that this construction is of particular value in pumps of the spill type in which the cylinders are filled through a slot or port opening in the side of the cylinder. When the pump is operating at less than maximum output displacement, the inlet port is uncovered during a portion of the reciprocatory motion of the piston and during this time it is possible for any u resonant pressure waves to be reiiected out through the inlet port into the Huid surrounding the cylinder block within the pump housing and such reflected waves which pass out through the inlet port can be effectively damped and broken up by the surrounding structure.

BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 is a longitudinal elevational cross-sectional view of a variable displacement axial piston pump according to the present invention;

FIGURE 2 i's an enlarged cross-sectional view of the reaction piston in the pump of FIGURE 1;

FIGURE 3 is a cross-sectional view of a reaction piston according to another embodiment of this invention; and

FIGURE 4 is a cross-sectional view of a reaction piston according to still another embodiment of this invention.

Referring now to the drawings in greater detail, the pump shown in FlGURE l includes a housing defining a fluid chamber 11 therein. At one end, the housing 10 and lluid chamber 11 are closed off by an end plate 13 and an end cap 14 which are secured together and to the housing by suitable means such as cap screws 16. The housing 10 is provided with an inlet opening 18 for connection to the* reservoir to insure a positive supply of fluid to the chamber 11 at all times.

At the other end of housing 10 is located the drive cam 20 which is rotatably journalled by a thrust bearing 22 and a radial roller bearing 23 in a bearing plate 25 mounted within the end of housing 10. The drive cam 20 has an axial bore 27 extending therethrough to receive a drive shaft 28 which it makes driving connection by means of the spline arrangement indicated at 29. The drive shaft 28 extends axially outwardly through an opening 31 in the end wall of housing 1t) and leakage of tluid at this point is prevented by means of a suitable seal assembly 33. It will be understood that the pump is operated by rotating the drive cam 20 by means of a suitable prime mover connected to the outer end of the drive shaft 23.

An axially extending generally tubular guide member 35 projects from the end plate 13 into the chamber 11 coaxially with the drive shaft 28. The guide member 35 has an enlarged bore 36 at the inner end thereof to receive a sleeve 38 which supports a spider plate 39 extending radially within the fluid chamber 11. On the other side of the spider plate 39 is securely mounted a bearing cage 41 to receive a suitable roller bearing assembly 42 to journal the inner end of drive shaft 28. A suitable retainer cup 45 is mounted on the end of the drive shaft to maintain the roller bearing assembly 4Z in position along the reduced diameter drive shaft end 43.

The drive cam 2l) is provided with a hub 47 projecting perpendicular to the inclined face 48 and a wobble plate 50 is rotatably mounted with respect to the drive cam by means of a thrust bearing assembly 51 and a roller bearing 52 between the wobble plate and the hub 47. The face of the wobble plate 50 away from the drive cam was provided with a plurality of recesses 54 for receiving the push rods as described in greater detail hereinafter. To prevent rotation of the wobble plate 50 with respect to the housing 10, the wobble plate has a radially extending stud 56 on which is mounted a bearing block 57 with fits within a channel guide 59 mounted within the housing 10. Since the bearing block 57 and stud 56 are confined to the longitudinal channel of the guide 59, the wobble plate' 5d does not rotate but because of the rotation of the inclined face 48 each portion of the wobble plate is progressively reciprocated along the axis of the drive shaft Z8 with a substantially sinusoidal motion.

A cylinder block 61 is slidably mounted on the guide member 35 for reciprocation to and from the end plate 13. Suitable means such as a key (not shown) are provided between the cylinder block and the guide member to allow the cylinder block to freely reciprocate but positively prevent any rotation of the cylinder block with respect to the end plate and the housing. The cylinder -block 61 is provided with a plurality of longitudinally extending cylinder bores 63, only one of which is shown for purposes of clarity. These cylinder bores extend from end to end through the cylinder block and are spaced equidistantly about the axis of the guide member and drive shaft and may be about tive or seven in number. An inlet port 65 is formed in the cylinder block 61 adjacent the mid point of each cylinder bore to admit iluid within the chamber 11 into the interior of the cylinder bore. A piston 67 is slidably journalled within the end of the cylinder bores 63 adjacent the wobble plate and extends through an opening in the spider plate 39. The piston 67 has a tubular skirt portion 63 extending adjacent the wobble plate 50 and a push rod 70 is mounted within the skirt portion to abut at the one end against the piston and at the other end to engage the recess 54 on the face of the wobble plate so that the cyclic motion of the wobble plate imparts a corresponding cyclic motion to the piston. A return spring 72 is mounted around the skirt 68 and abuts at its one end against the spider plate 39 and at the other end against a radially extending flange 73 on the end of the piston skirt 68. Thus, as the push rod serves to move the piston toward the left as seen in FIGURE 1, the return spring 72 provides a biasing force toward the right to maintain the push rod 70 at all times in positive contact with both the piston and the wobble plate recess 54.

In order to conduct the fluid from the cylinder bores 63 as well as to eliminate any fluid pressure reaction forces acting on the cylinder block 61, a reaction piston 75 is slidably received within the other end of the cylinder bore 63. The reaction piston '75 is tubular to conduct the liuid from the cylinder bore 63 and extends beyond the end of the cylinder block 61 through an opening in a retainer plate 77 secured to the inner side of end plate 13. The reaction piston 75 extends into a recess 78 formed in the end plate 13 and in general alignment with the recess 73 is a bore 81 formed in the end cap 14. A check valve member S2 is mounted within the bore 81 and projects through the end plate 13 into the recess 78 where it makes sealing engagement with the end face of the reaction piston 75. A spring 84 surrounds the end portion of reaction piston 75 to abut at the one end against a snap ring 35 and at the other end against the retainer plate 77 so as to provide a biasing force urging the reaction piston 75 into sealing engagement with the check valve member SZ. Within the bore 31 is a check valve plunger 87 biased by a spring 88 against the other end of the valve check member 82, so that the check valve plunger 87 is opened only when the fluid within the cylinder bore 63 and the reaction piston 75 is pressurized by the piston 67, and in this case upon the movement of the piston in the forward or pumping direction, the fluid Within the cylinder bore is pumped outward past the check valve plunger S7 and through a passage 91 into a pump outlet 92. As previously stated, it will be understood that the pump has a plurality of cylinders and pistons and the pump outlet 92 is connected to each of the associated check valves so as to receive the total fluid from all of the cylinders of the pump.

The pump of FIGURE l is of the variable displacement type operating under what is known as spill control. When the piston 67 is on the rearward stroke, it uncovers the inlet port 65 so that the fluid within the fluid chamber 11 can flow through this inlet port to completely fill the cylinder bore 63. As the piston 67 starts toward the left on its forward stroke, the fluid is initially forced back out through the inlet port 65 into the chamber 11. However, after the piston 67 has moved a suflicient distance to seal olf the inlet port 65, fluid can no longer escape in this manner and until the piston reaches the end of the stroke, the fluid then passes outward through the check valve into the pump outlet 92. From this, it will be seen that the position of the inlet port 65 with respect to the stroke of the piston 67 determines the effective outlet volume of the pump. When the cylinder block 61 is in the position shown in FIGURE 1 the pump is effectively at its maximum output displacement with the inlet port 65 uncovered only for a sullicient length of time to allow complete filling of the cylinder bore 63. It will be seen that if the cylinder block 61 is moved toward the left this will move the inlet port 65 toward the left with respect to the position of the piston 67 so that fluid within the cylinder bore will be forced back outward through the inlet port 65 for most of the stroke of the piston. Thus, it takes a longerportion of the forward stroke of the piston before the inlet port 65 is sealed off and hence a lesser amount of fluid is pumped outward into the pump outlet 92.

While it is possible to utilize the pump outlet pressure for actuating a fluid motor to shift thecylinder block it is also possible to use a simple manual control as shown in FIGURE 1 in which the cylinder block 61 is provided with a slot 94 which is engaged by -a radially projecting finger 95 on .an axially sliding rod 97. The rod 97 can be moved within the housing by means of a screw 98 mounted on the end cap 14 and rotated by suitable means such as a hand wheel 99 in order to shiftrthe cylinder block to vary the pump output displacement.

In prior art pumps of the type described above, the reaction piston 75 has generally been in the form of a tube having a bore of substantially uniform diameter throughout its entire length and with this bore having a maximum diameter as to prevent any substantial restriction of fluid flow between the cylinder bore and the check v-alve assembly. In general the reaction piston was chosen to have a sufficient wall thickness to provide suicient biasing force `as a result of the fluid pressure within the cylinder bore acting on the effective cross sectional area of the reaction piston to provide a tight seal Vagainst the check valve member since the spring 84 is generally quite light and serves primarily to provide `a seal under start-up conditions and Whenever the demands on the outlet of the pump .are such that very little pressure is built up within the cylinder bore.

However, according to the present invention it has been found that if the reaction piston 75 is constructed so as to provide a progressively decreasing diameter to the iluid passing through it, a substantial reduction in noise can be achieved. This progressive decrease in diameter can be accomplished in a number of different -ways as shown in FIGURES 2, 3 and 4. In FIGURE 2 the reaction piston 75 is formed at the end within the cylinder bore 63 with a conical portion 102. This conical portion 102 at its inner end joins a first cylindrical portion 103 which terminates in a shoulded 104 adjacent a second cylindrical portion 106 of reduced diameter. The second cylindrical portion 106 extends for a slightly greater distance than the first cylindrical portion 103 and terminates in a second shoulder 4107 with a third cylindrical portion 109 which, again, is of slightly greater length than the two preceding cylindrical portions. This cylindrical portion again terminates in still a third shoulder 111 Iwhich joins `a reduced diameter portion 113 which extends to the end face 114 of the reaction piston.

The embodiment of FIGURE 3 is somewhat different but again has a conical portion 116 together with rst, second and third cylindrical portions 117, 119 and 121 respectively which terminate in iirst ,second and third shoulders 116, 120 and 122, respectively. Each of these cylindrical portions is of substantially shorter length than in the embodiment of FIGURE 2, but each smaller cylindrical portion is of slightly greater length.

The third shoulder 122 joins a reduced diameter portion 124 of relatively short length at about the mid-point of the reaction piston, and the remaining portion of the passage forms .a conical portion 126 of progressively increasing diameter toward the end face 127.

In the embodiment of FIGURE 4, the stepwise arrangement of FIGURES 2 and 3 is not used and instead the reaction piston has a smooth surface 131 extending from its greatest diameter at the mouth 129 to the rcduced end portion 133 adjacent the end face .134. In this arrangement the surface 131 is in the form of a surface of revolution corresponding to a negative exponential horn which reduces in diameter away from the end within the cylinder bore. In the embodiments of FIGURES 2 and 3 the cylindrical portions and shoulders .are chosen with dimensions to provide an approximation of the negative exponential horn and in the arrangement of FIGURE 3, the horn is shortened and the conical portion aids in the proper formation of a venturi at the reduced diameter 124 so as to allow the fluid which reaches the maximum velocity of reduced portion to decrease in velocity as it approaches the check valve assembly.

With the reaction piston of this invention, it will be seen that low frequency standing Waves are more readily broken up than in the case of a reaction piston having a bore of uniform diameter. Thus, with the diameter of the reaction piston decreasing away from the piston head, this surface of the reaction piston in reEect provided a plurality of reflecting surfaces instead of a single surface at the end of the reaction piston so that the energy in the standing wave Within the cylinder and reaction piston is split up into .a number of different frequencies which under certain conditions, depending upon the position of the piston within the cylinder bore, may tend to cancel each other. Since some of the surfaces are located much closer to the piston head, the resulting frequencies of some portions of the resultant noise yare in a rather high range where they are more easily damped. It should be understood that because the piston is moving, the effective length of the resonant cavity is continually changing while the pump is running.

Another reason why the acoustic noise level is decreased with the reaction piston of this invention is that since the effective diameter of the reaction piston is progressively decreasing, thereby progressively decreasing the effective area, it is necessary that the velocity of the uid passing therethrough increase a corresponding amount. Thus, since the fluid passing the reflective areas in the reaction piston is moving at varying velocities, the fact that the iluid may be moving at various velocities near a reflective surface tends to break up the reflective waves and change their frequencies.

This arrangement of the reaction piston also serves to reduce the noise level when the pump is operating at less than maximum output volume since during the time when the inlet port is uncovered the acoustic waves pass outward through the inlet port into the surrounding fluid chamber where they can be more effectively damped by the remainder of the pump structure and the fluid within the pump. Although this occurs to some extent using a reaction piston of uniform diameter, all the acoustic energy tends to be at a given frequency at any given instant corresponding to a given piston position, whereas the reaction piston structure of this invention requires that any standing waves generated be at a number of different frequencies, particularly at higher frequencies where the pump structure has a greater dampening effect.

The negative exponential horn of FIGURE 4 should be considered an idealized representation which produces the maximum desired effect but which may not be readily practicable because it is extremely expensive to reproduce with sufficient accuracy. With the arrangements of FIG- URES 2 and 3, the shape of the negative exponential horn is effectively achieved by simple cylindrical portions of different diameters which are easily reproduced. Of course, varying numbers of steps can be used and an increased number of steps will result in a closer approximation to the shape of FIGURE 4.

Because of the fluid in the arrangements of FIGURES 2 and 4 reaches a maximum velocity adjacent the end face of the reaction piston, this fluid therefore has a high velocity as it reaches the check valve assembly. Under certain conditions, this may be extremely undesirable because it may tend to cause cavitation and severe wear eifects upon the check valve assembly, and accordingly it is possible to resort to the arrangement of FIGURE 3 in which the reduced diameter portion producing the venturi effect is adjacent the mid portion of the reaction piston and the increasing diameter of the conical portion 126 allows the fluid to reduce its velocity as it reaches the check valve assembly.

While several embodiments of this invention have been shown and described in detail, it is recognized that the invention is not limited to the particular forms shown.

What is claimed is:

1. A pump comprising a cylinder block, a cylinder bore in said cylinder block, a piston reciprocable in one end of said cylinder bore, an inlet to said cylinder bore, a reaction piston extending into the other end of said cylinder bore, a check valve assembly adjacent the end of said reaction piston away from said pumping piston, said reaction piston having a passage therethrough extending between said cylinder bore and said check valve assembly, said passage providing portions of progressively decreasing cross-sectional area away from said cylinder bore toward said check valve assembly.

2. A pump as set forth in claim 1 wherein said reaction piston passage is coaxial with said cylinder bore.

3. A pump as set forth in claim 2 wherein said portions of said outlet passage of progressively decreasing dross-sectional area comprise a plurality of cylindrical portions interspaced by shoulders, each of said cylindrical portions being of progressively smaller diameter and being progressively longer in length.

4. -A pump as set forth in claim 2 wherein said reaction piston passage is formed with a smooth internal surface in the form of a negative exponential curve rotated about the axis of said passage.

5. A pump as set forth in claim 3 wherein the cylindrical portion of smallest diameter is intermediate the ends of said reaction piston and said passage includes a portion of increasing cross-sectional area away from said smallest cylindrical portion toward said check valve assembly.

6. A pump comprising a housing defining a iluid chamber, a cylinder block mounted on said housing within said uid chamber for axial movement therein, means to move said cylinder block axially within said chamber, a plurality of cylinder bores in said cylinder block extending from end to end therethrough, a pumping piston extending into one end of each of said cylinder bores, drive means to progressively reciprocate said pumping pistons, a reaction piston extending into the other end of each of said cylinder bores, each reaction piston having a passage therein to conduct tluid from said cylinder bore, each of said cylinder bores being provided with a uid inlet intermediate the ends thereof between said pumping piston and said reaction piston, an outlet manifold on said housing, check valve means interconnecting each of said reaction pistons with said outlet manifold, said reaction piston passage having portions at varying distances from said pumping piston with each of said portions having a progressively decreasing cross-sectional area away from said pumping piston whereby said portions of progressively decreasing cross-sectional area function to reduce the hydraulic noise produced by the pump.

7. A pump as set forth in claim 6 wherein said portions of said outlet passage of progressively decreasing cross-sectional area comprise a plurality of cylindrical portions interspaced by shoulders, each of said cylindrical portions being of progressively smaller diameter and being progressively longer in length to approximate the form of a negative exponential horn.

8. A pump as set forth in claim 6 wherein said reaction piston passage is formed with a smooth internal surface in the form of a negative exponential horn.

9. A pump as set forth in claim 7 wherein the cylindrical portion of smallest diameter is intermediate the ends of said reaction piston and said passage includes a portion of increasing cross-sectional area away from said smallest cylindrical portion toward said check valve assembly to decrease the uid velocity at said check valve assembly.

References Cited UNITED STATES PATENTS 2,136,098 10/1938 Browne 230-232 X 2,672,159 3/1954 Walton 138-44 2,790,463 4/ 1957 Delano et al 138-44 2,904,076 9/1959 Engel et al 13S-44 X 3,146,798 9/1964 Chenault 138-44 3,007,420 11/1961 Budzielt 103--162 FOREIGN PATENTS 901,268 1/ 1959 Great Britain.

WILLIAM L. FREI-3H, Primary Examiner U.S. Cl. X.R. 

